An apparatus for measuring the thickness of thin layers includes a probe, guide means for gently setting down said probe perpendicularly on a layer and an electric line connected to said probe. Such layer thicknesses typically lie between 2 and 10 micrometers, but may also reach 100 micrometers. Measuring inner layers of tubular articles, such as for example toothpaste tubes, paint tubes, hair-spray cans, preserved-food cans or the like, presents difficulties. All these consumer articles have in common that their wall has to be protected toward the inside by a layer. The wall on tubes is often aluminum, which is virtually pure aluminum. Sometimes, however, the wall is also of a ferrous metal. Sometimes, however, even the material itself produces protective layers, such as for example the oxide layer on light metals. It is known that the magnetic measuring principle is used if the base material is a ferrous metal. Otherwise, the magnetic principle may also be used. Sometimes, however, it is also required to measure wall thicknesses, such as for example aluminum wall thicknesses, without said walls having an insulating layer or a protective layer. In some cases, it is better to measure from inside to outside, instead of from outside to inside. The coating layers may also be layers which are intended to insulate electrically. The thin layers may, however, also be of quartz, bitumen or some other thin layer.
In particular in those articles which have a diameter in the lower cm range, until now it was scarcely possible to provide information on the thin layer on the inside. Added to this is the fact that, in the case of such thin layers, there is the risk that they may be influenced in their thickness by the application of the probe. This may vary from piercing the layer to crater-like depressions, as are known for example from hardness measurements. There it is in actual fact necessary to press a test piece into the layer, whereas in fact this is to be avoided here.